in my head sirius and regulus are zuko and azula in another universe. The heirs of a kingdom, the familial expectations, groomed from a young age by their families, seperated by those expectations. Sirius being the "different" outcasted one, regulus being the "prodigy" who couldn't unlearn the family's ways. zuko's exile (leaving home at 16), azula's crashout when the life she wanted isn't what it seemed (sets the stage for regulus' betrayal). Zuko joining team avatar AKA sirius finding the marauders. Yup.